The present invention relates to a new method for the preparation of poly(silyl ester)s. The invention further relates to novel poly(silyl ester)s, and in another aspect, the invention further relates to the use of hydrolysable poly(silyl ester)s where the use of hydrolysable silyl ester groups is advantageous.
Poly(silyl ester)s possess a variable yet predictable degradation behaviour and as such, have a broad range of potential applications wherever the presence of hydrolysable groups is advantageous. For instance, it is known to use degradable polymers in general in the medical, environmental, biomedical, and agricultural areas, wherein the ability of the polymers to break down into biologically or environmentally resorbable small-molecule byproducts is of great use. WO 03/105920 discloses the use of polymer containing coating compositions for medical implant devices. Poly(silyl ester) compositions are believed by the present inventors to represent viable alternatives to the compositions disclosed therein.
Another application is as a resin or co-resin for self-polishing antifouling paints, for instance, as binders for modern antifouling coatings, although the use of hydrolysable poly(silyl ester)s, as noted above, will have many applications where the fact that the degradation behaviour can be affected by the nature of the functionality attached to the silicon atoms would be a positive advantage.
Trialkylsilylcarboxylates of aliphatic carboxylic acids can be obtained by transesterification. H. H. Anderson et al describe in J. Org. Chem 1716 (1953) the reactions of triethyl silyl acetates and diethyl silyl diacetates with halogenated propionic acids and in J. Org. Chem. 1296 (1954) the reactions of dimethylsilyl di(trifluoro acetate) or dimethylsilyl dipropionate with chloroacetic acid; they distill the acetic, propionic or trifluoroacetic acid under reduced pressure.
Russian chemists (Izv.Akad.Nauk.Ussr.Ser.Khim. 968 (1957)) run similar reactions but at much higher temperatures (190-210° C.)
JP 95070152 A discloses reactions of trialkylsilylacetates with C6 to C30 carboxylic acids (e.g. palmitic, myristic, benzoic, . . . ); the acetic acid is distilled under reduced pressure or azeotropically with hexane.
Poly(silyl ester)s are characterised by the fact that they comprise more than one silyl carboxylate unit in the oligomeric/polymeric backbone. In other words, poly(silyl ester)s contain —Si—O—C(O)— linkages along the polymer backbone, and are a class of degradable polymer systems with a variable, yet predictable, degradation behaviour.
Silyl ester functionalities have been prepared by many routes and therefore it may have been envisaged that the synthesis of poly(silyl ester)s would be relatively straightforward. However, problems such as salt formation, side reactions, monomer insolubility/impurity, and/or incomplete reaction, ensured that many of the envisaged polymer syntheses were not in fact suitable. Indeed, it is only recently that the incorporation of acyloxysilane groups in a polymer backbone has actually been achieved.
Wooley et al have developed and disclosed synthetic routes for the preparation of poly(silyl ester)s, including transsilylation esterification of AA/BB comonomers (see Macromolecules (1995) 28 8887; Macromolecules (1998) 31 7606; J. Polym. Sci., Part A Polym. Chem. (1999) 37 3606, Macromolecules (1998), 31 15; and J. Organomet. Chem. (1998). 542 235), transsilylation esterification of AB monomers (see Macromolecules (2000) 33 734; and J. Organomet. Chem. (1998) 542 235), hydrosilylation of AB monomers (see Macromolecules (2000) 33 734), and cross-dehydrocoupling polymerisation of AB and AB2 monomers (see Macromolecules (2001) 34 3215, and references cited therein).
For example, the transilylation ester interchange reaction of chlorosilanes with TMS-blocked silyl esters at temperatures in the range of 100-135° C. for 10-14 days, leading to the formation of corresponding polymers with the concomitant distillation of trimethylsilyl chloride, using, for example, N,N-dimethylformamide (DMF) as a catalyst, has been described by Wooley et al, in Macromolecules (1998) 31 15. This method does have its advantages; no base is required, and the trimethylsilyl chloride by product is volatile. Moreover, the reaction proceeds at relatively low temperatures, and without the addition of a solvent. However, TMS-blocked silyl esters are expensive reagents and the release of trimethylsilyl chloride is harmful to the environment.
Another method described by Wooley et al in Macromolecules (2000) 33 734 and Macromolecules (2001) 34 3215 is the polymerisation accomplished via hydrosilylation between a silyl hydride function and a carbon/carbon double bond or a carboxylic acid function present in the same molecule (AB monomer system) or in different molecules (AA/BB monomer system). This method suffers from the disadvantages of requiring the use of transition metal catalysts such as platinum or, palladium derivatives (Pt(COD)Cl2; Pd/C) in order to generate hydrogen when carboxylic acid functions are involved and the limited accessibility of the starting materials.
Therefore, there remains the need to find a novel method for the preparation of poly(silyl ester)s avoiding or at least alleviating the aforementioned problems associated with the prior art methods of synthesising hydrolyzable poly(silyl ester)s.